This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to depositing electrically conductive layers on a substrate.
There are a variety of methods by which packaged integrated circuits are attached and electrically connected to printed circuit boards. One such method is the ball grid array. In a ball grid array, sets of electrically conductive contact pads are formed on each of the mating surfaces of a package substrate and the printed circuit board. Electrically conductive balls, such as solder balls, are then positioned on one or the other or both of the sets of electrically conductive contacts pads. The mating surfaces of the package substrate and the printed circuit board are brought into close proximity one with the other such that the solder balls contact both sets of contact pads. Finally, the assembly is heated to flow the solder balls so that they form electrical connections between individual ones of the contact pads on both the package substrate and the printed circuit board. The assembly may then be selectively potted with an insulating material, such as a filled adhesive, to fill in the interstitial spaces between the solder balls, and elsewhere, to protect and further attach the package substrate to the printed circuit board.
A gall grid array tends to provide good adhesion between the package substrate and the printed circuit board. However, some applications are more demanding than others in the degree of bonding strength that is required between the package substrate and the printed circuit board. In other words, in some applications a great degree of bonding strength is required or the package substrate breaks away from the printed circuit board to some degree. While such a break may not be complete, in that the package substrate falls away from the printed circuit board, even breaks of a lesser degree may be sufficient to cause failure of the assembly, if the breaks cause a loss of electrical continuity through one or more of the bonding padxe2x80x94solder ballxe2x80x94bonding pad connections. Thus, it is desirable to have as great a degree of strength in the connections as possible, given the other constraints in the manufacturing process, such as processing and tooling costs.
What is needed, therefore, is a connection that provides good bonding strength without excessive costs.
The above and other needs are met by a method of forming an electrical contact on a surface of a substrate. A first layer of a first electrically conductive material is formed on the surface of the substrate, where the first layer is formed in a substantially contiguous sheet across the surface of the substrate. A non electrically conductive masking layer is applied to the first layer, where the masking layer leaves exposed first portions of the first layer and covers second portions of the first layer. The substrate is immersed in a first electrolytic plating bath, and a first electrical potential is applied between the first layer and the first electrolytic plating bath, thereby causing the formation of a second layer of a second electrically conductive material on the exposed first portions of the first layer. The substrate is immersed in a second electrolytic plating bath, and a second electrical potential is applied between the first layer and the second electrolytic plating bath, thereby causing the formation of a third layer of a third electrically conductive material on the second layer. The masking layer is removed from the substrate to expose the second portions of the first layer, and the exposed second portions of the first layer are removed to form discrete contact pads from the first portions of the first layer and the overlying second layer and third layer.
In this manner, a very strong bonding pad is formed, which is highly resistant to delamination and other failures from shearing and tensile forces. The electrolytic deposit of the second and third layers tends to create a very strong bond between the first, second, and third layers. Previously, such layers were deposited by electroless or autocatalytic processes. The impurities in such processes tended to weaken the attractive forces between the layers, and thus bonding pads formed during such processes tend to delaminate or otherwise fail easier than those formed by the process of the present invention.
In various preferred embodiments of the invention, the first electrically conductive material is copper, the second electrically conductive material is nickel, the third electrically conductive material is gold, and the non electrically conductive masking layer is photoresist. Most preferably the step of applying the non electrically conductive masking layer to the first layer is accomplished by coating the first layer with photoresist, soft baking the photoresist, exposing a pattern in the photoresist, developing the pattern in the photoresist, and hard baking the photoresist. The step of removing the masking layer from the substrate is preferably accomplished by ashing the masking layer.
According to another aspect of the invention, there is disclosed herein a package substrate having an electrical contact formed by the method of claim 1.